Question: Q. 15. Give reasons for the following :
(i) A photodiode, when used as a detector of optical signals is operated under reverse bias.
(ii) The band gap of the semiconductor used for fabrication of visible LED’s must at least be $1.8 \mathrm{eV}$.
A&E [CBSE SQP 2014]
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Solution:
Ans. (i) When operated under reverse bias, the photodiode can detect the changes in current with the changes in light intensity more easily. $\mathbf{1} \frac{1}{2}$
(ii) The photon energy, of visible light photons varies from about $1.8 \mathrm{eV}$ to $3 \mathrm{eV}$. Hence for visible LED’s, the semiconductor must have a band gap of $1.8 \mathrm{eV}$.
[CBSE Marking Scheme 2014] 1½
Long Answer Type Questions
(5 marks each)
